Fungi to work with

In the third of a six article series on the produce world’s very own natural born killers, Horticulture Research International examines biological control with insect pathogenic fungi.

Farmers and growers are under pressure to reduce the use of chemical pesticides, but at the same time the control of insect and mite pests is becoming increasingly difficult due to pesticide resistance and the decreasing availability of products.

Alternative control methods are needed urgently. Many growers are now familiar with the use of predators and parasitoids for biological control, but pests can also be managed using specific micro-organisms that kill insects. These include insect pathogenic fungi, which are widespread in the natural environment and cause infections in many pest species.

More than 750 species of insect pathogenic fungi are known to science. Overall they attack a wide range of insect and mites, but individual species and strains of fungus are very specific. The fungi produce spores which infect their host insect by germinating on its surface and then growing into its body. Death takes between four and 10 days, depending on the type of fungus and the number of infecting spores. After death, the fungus produces thousands of new spores on the dead body, which disperse and continue their life cycle on new hosts.

The spores of many fungal pathogens can be mass produced on simple culture media such as grain. This means that they can be used against pests in much the same way as conventional pesticides. Pathogens used in this way are called biopesticides.

There are other species of fungi that cannot be mass produced, but which cause natural epidemics. In some cases it is possible to exploit these for cost-free biological control.

The insect pathogenic fungi used for biological control have a range of desirable characteristics including safety to people, compatibility with other natural enemies, and a lack of toxic residues. They also offer the possibility of providing persistent control by multiplying in the pest population. Because they have contact action, they are good for the control of sap-feeding pests, like aphids and whiteflies, which cannot be infected by other types of biopesticide (such as bacteria and viruses) which are active only when ingested.

The downside is that they are more expensive than chemicals and can be affected adversely by certain environmental conditions. There can also be a false expectation that biopesticides will have the same efficacy as chemical insecticides, while their desirable biological qualities are overlooked.

In addition, the costs of registration in the UK are high in relation to the current market size, deterring companies from pursuing registration. At present only two commercial products are available to UK growers, namely Vertalec (for control of glasshouse aphids) and Mycotal (for glasshouse whitefly). Both are based on the fungus Verticillium lecanii and are produced by Koppert, but were originally developed at the Glasshouse Crops Research Institute at Littlehampton before the research was transferred to Warwick HRI.

More commercial products are available to growers in Europe and the US, some of which may become available in the UK in future.

The scientists at Warwick HRI have a long history of research on insect pathogenic fungi, much of it in collaboration with other UK organisations. For example, in Defra- funded work with Rothamsted Research, we are investigating fungi as control agents of the varroa mite, which has devastated populations of honey bees around the world.

We have found that varroa is very susceptible to fungal pathogens, and we have identified fungal strains that infect varroa but have little effect on bees or other non target insects. In due course it may be possible to develop one of these strains into a commercial control agent.

Glasshouses are some of the best environments for insect pathogenic fungi, because the temperature and humidity are favourable for fungal infection. In HDC-funded work with Stockbridge Technology Centre, we found that commercial agents, based on the fungus Beauveria bassiana, can give very good control of western flower thrips on cucumber and red spider mite on tomato, and they are also compatible with the main predators used against these pests.

Because it is fast acting, the fungus makes a good supplementary control agent, providing rapid back-up at times when the pest starts to outpace introduced predators. In current Defra- funded work with ADAS, Keele University, and Rothamsted Research, we are looking at ways to improve the efficacy of Beauveria bassiana-based fungal biopesticides against western flower thrips by exploiting new information on thrips behaviour.

Unfortunately, although these Beauveria bassiana products are available in the US and some European countries, they are not available currently in the UK. Outdoor crops present a much harsher environment for insect pathogenic fungi, and research on their use in field crops in the UK is still in its infancy.

In HDC and Defra-funded work, we have recently begun to investigate commercial fungal control agents for the control of aphids on lettuce and brassicas. These agents worked very well against aphids in laboratory experiments. In field experiments, we assessed a Beauveria bassiana product against cabbage aphid, peach potato aphid, and currant-lettuce aphid. Although we were able to reduce the development of cabbage aphid populations, performance against the other two aphid species was disappointing.

This may well be due to the difficulty of contacting these species using foliar sprays and adverse effects of the weather on fungal performance. Further work is needed to investigate these factors.

While these research developments are very exciting, it should be remembered that insect pathogenic fungi do not provide ‘magic bullet’ solutions to pest control, and they will only be effective if used as part of an integrated pest management approach.

One of the biggest stumbling blocks at the moment is the difficulty of making commercial products more widely available to growers. This is caused partly by the UK and EU regulatory systems, which were put in place to deal with chemical pesticides, but now have to deal with biopesticides and other alternative control agents.

We have started working recently with experts from the politics department at Warwick University to consider ways in which regulatory systems can be adjusted to take account of the need for innovation in the way that we control pests, whilst still protecting health and safety. Developments in this area could make sustainable approaches to pest control more accessible to the UK fresh produce industry.